This invention relates to actuators for moving loads, for example actuators for subsea downhole chokes.
Such actuators are normally employed in subsea environments in order to provide control of chokes in downhole fluid production systems. Such chokes control the flow of fluid into or out from the well bore. Conventionally, such chokes have been simple on/off devices that merely fully opened or fully closed the flowline. Recently, there has been a requirement for variable flow control, generally in a limited number of flow control steps. The control has needed to be sufficiently accurate in order to balance pressures between different fluid sources feeding a well bore or within a multiplicity of branches of a well bore. Furthermore, the actuator mechanism has needed to be self-locking in position in the event of power failure or emergency situation.
Conventionally, a piston type of hydraulic actuator has been employed. A problem which may be associated with such actuators is that the step positions achievable by means of the hydraulic drive may be relatively crude in accuracy. Furthermore, this type of actuator does not have an inherent self-locking characteristic in the event of power failure. It has been proposed to provide an electrical backup operation which, in the event of power failure, forces the output of the actuator into a predetermined safety position. This has typically resulted in the prevention of further operation of the primary hydraulic drive after use of the electric drive. Therefore, such proposals have tended to be used as one-off emergency mechanisms, requiring expensive replacement and/or manual resetting of the actuator after recovery from the emergency.
The invention provides an actuator for moving a load, the actuator comprising an output region arranged to engage the load and first and second rotary motor means, the actuator being arranged so that, in a first mode of operation, the first rotary motor means imparts linear motion to the output region via a first screw connection; and, in a second mode of operation, the second rotary motor means imparts linear motion to the output region via a second screw connection.
The provision of screw connections permits gradual linear motion to be imparted to the output, to enable more accurate position control of the load. The provision of two rotary motor means permits the continuing performance of the actuator in the event of failure of one of the motor means.
The invention finds particular application in controlling the sleeves of subsea choke devices.
Preferably the actuator is arranged so that, in the first mode of operation, the first screw connection is urged against an intermediary-member carried by the second screw connection. In this manner, linear motion is imparted to the output region.
There may also be a third mode of operation, wherein the output means is driveable linearly by operation of both the first and second motors, via their respective screw connections. In this mode of operation, extra force is imparted to the output region, and so is suitable for emergency operation.
Preferably, the first and second screw connections are aligned axially so that maximum force is imparted to the output region.
One of the motor means may be electric, the other hydraulic. Alternatively, both motor means may be hydraulic or electric.
Either or both of the motor means may comprise two rotary motors. In this instance, one of the motors would be arranged to effect linear motion of the output region in a first direction. The other motor would be arranged to produce linear motion of the output region in a second direction.
At least one of the screw connections may comprise a nut carrying a carriage. The actuator may also include guides for the carriage. At least one of the screw connections may be arranged to carry a splined shaft.